FIGS. 18A-18C show a schematic block diagram of a conventional vibrating-type level sensor disclosed in Japanese Patent Laying-Open No. 11-351944. Referring to FIG. 18A, a detection pipe unit 1 has its base 11 serving as a fixed end and its leading end closed with a closing unit 12 to serve as a free end, and accordingly constitutes a folded cantilever. A thin rectangular vibrating plate 2 is provided within detection pipe unit 1. Specifically, one end of vibrating plate 2 is fastened to closing unit 12 of detection pipe unit 1 and the other end thereof is provided with a permanent magnet 3 to serve as a free end.
In addition, an electromagnet 4 is attached closely to the inner wall of detection pipe unit 1 to face vibrating plate 2 in the direction of axis thereof. Electromagnet 4 is driven by an alternating current to generate a magnetic field. Between this magnetic field and a magnetic field of permanent magnet 3, an attracting/repelling action is produced to cause oscillations of the folded cantilever having vibrating plate 2, closing unit 12 and detecting pipe 1 with base 11 serving as the fixed end.
A distortion detecting element 5 is provided on the inner wall at base 11 of detection pipe unit 1. Distortion detecting element 5 detects the state of oscillation amplitude at base 11 of detection pipe 1 to convert the oscillation amplitude into an electric signal and supply the electric signal to an amplifier circuit 6. Amplifier circuit 6 amplifies the supplied signal and provides the amplified signal again to electromagnet 4.
Suppose that there is a relation as shown in FIG. 18B between the polarity of current applied to electromagnet 4 and a magnetic field generated at electromagnet 4. Then, the pole of electromagnet 4 that faces permanent magnet 3 is the north pole and thus attracting force is generated between this north pole and the south pole of permanent magnet 3 attached to vibrating plate 2 while repelling force is generated between the north pole of permanent magnet 3 and the north pole. Consequently, the free end of vibrating plate 2 is forced and displaced upward in FIG. 18B.
On the contrary, suppose that the current applied to electromagnet 4 has the opposite polarity. Then, as shown in FIG. 18C, the pole of electromagnet 4 that faces permanent magnet 3 is of the opposite polarity, i.e., the south pole. Accordingly, this south pole repels the south pole of the permanent magnet of vibrating plate 2 while the south pole and the north pole of the permanent magnet attract each other so that the free end of vibrating plate 2 is forced downward to cause change in the oscillation state. In this way, the polarity of current applied to electromagnet 4 may be changed according to the natural frequency of the vibrating system of the folded cantilever to produce and sustain oscillations.
According to the example shown in FIGS. 18A-18C, oscillations of the vibrating system are detected by detecting element 5 and then converted into an electric signal which is amplified by amplifier circuit 6 and supplied again to electromagnet 4, while a detecting circuit 7 outputs a detection signal. A piezoelectric element or acceleration pickup may be employed as oscillation detecting element 5. The piezoelectric element, however, has problems in that the piezoelectric element is brittle, the piezoelectric element attached to the detection pipe with an adhesive is susceptible to environment and temperature characteristics, and thus the reliability of the piezoelectric element itself is low.
Instead, a method may be used with a detecting circuit disclosed in Japanese Patent Laying-Open No. 5-87612 for example. According to this method, a phase comparator circuit, a loop filter (integrating circuit) and a voltage-controlled oscillator circuit (VCO circuit) constitute a PLL circuit. Pre-oscillator circuit generates an oscillation frequency according to an object to be detected, and the oscillation frequency is supplied to the phase comparator circuit of the PLL circuit. The oscillation frequency is then compared with a frequency signal of the VCO circuit to detect the object.
As for the above-described conventional examples, however, the detecting circuit has a relatively larger number of components, resulting in problems of greater cost, increased complexity of the structure and increase in the number of assembly steps. Such a large number of components also results in deterioration in reliability.
One chief object of the present invention is therefore to provide a method of detecting the impedance of an oscillation coil in a vibrating-type level sensor, according to which a beat voltage is detected that is caused by interference of an applied voltage and a counter-electromotive force, the applied voltage being generated when a frequency signal is swept at a rapid rate, and the magnitude of the beat voltage is used to determine whether there is a particulate matter or not, and to provide a method and a device for detecting an object according to the impedance detecting method.